Love Proxmox, it's done everything I needed of it.
I don't use it to anywhere near it's potential, but I can vouch for the robustness of its backup process. I've had to restore more than handful of VMs for various reasons, and they've all been rock-solid upon restoration. I would like to use its high-availability features, but haven't needed them and don't really have the time to tinker so much these days.